Apparatus for evaluation of coated parts

ABSTRACT

A bending test fixture for test coupons includes a test region for receiving a test coupon and first and second supports on opposed sides of the test region. One of the first or second supports includes a first pair of spaced-apart support points and the other of the first or second supports includes a second pair of spaced-apart support points that are narrowly spaced relative to the first pair of spaced-apart support points. There is a load shaft for applying a load to the second support and an articulated joint between the load shaft and the second support for uniformly distributing the load to the first support.

REFERENCE TO RELATED APPLICATIONS

The present invention is a division of U.S. patent application Ser. No.12/336,833, filed Dec. 17, 2008.

BACKGROUND OF THE INVENTION

This disclosure relates to a method and fixture for non-destructivelyverifying performance of a coated part. Hard coatings are typicallyapplied to external surfaces of parts to enhance wear resistance,erosion resistance, or other properties of the part. The hard coatingmay debit other properties of the part, such as fatigue performanceunder cyclical loading conditions.

Manufacturers, end-users, or the like may verify fatigue performance ofcoated parts. For instance, sample parts may be taken from a group ofparts and fatigue tested as indication of whether the other parts in thegroup have a desired level of fatigue performance. One drawback of thismethodology is that the tested parts are effectively destroyed in thetesting process, which may add expense to the overall manufacturingprocess.

Another methodology involves indirectly estimating the fatigueperformance from a measured property of sample parts or couponsrepresenting sample parts. For instance, coating hardness, modulus ofelasticity, or residual stress may be used individually or incombination to estimate a level of expected fatigue performance. Theestimation may be based on prior-collected experimental data thatestablishes a correlation between these properties and fatigueperformance. The drawback here is that these properties do not indicatethe mechanical strength of the material deposited, but rather aconsistency in the plating process which one would suppose wouldindicate a mechanical strength consistency. Additionally some of thesetest methods have complexities on their own in how a measurement ismade. Additionally, there are questions as to whether these propertiesare reliable predictors of fatigue performance.

Conventional methodologies for verifying fatigue performance also lackflexibility for variations of the coated parts, such as coatingthickness (e.g., different models, part numbers, etc.). For example, aparticular hardness, modulus, and residual stress that corresponds to adesired fatigue performance for a coating having a given thickness maynot correspond to the desired fatigue performance level if the coatinghas a different thickness. A new correlation between the coating of thedifferent thickness and the fatigue performance would have to beexperimentally established.

SUMMARY OF THE INVENTION

An example bending test fixture for the test coupons includes a testregion for receiving the test coupon, and first and second supports onopposed sides of the test region. One of the first or second supportsincludes a first pair of spaced-apart support points and the other ofthe first or second supports includes a second pair of spaced-apartsupport points that are narrowly spaced-apart relative to the firstpair. A load shaft applies a load to the first support, and anarticulated joint between the load shaft and the first support uniformlydistributes the load to the first support.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the disclosed examples willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 is an example method of non-destructively verifying performanceof a coated part.

FIG. 2 is an example of a bending test fixture for test coupons.

FIG. 3 is an alternate view of the bending test fixture of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates an example method 20 of non-destructively verifyingperformance of a coating. It is to be understood that the method 20 isnot limited to any particular type of coated part and may be appliedparts coated using any deposition process, such as electroplating,plasma-spraying, and the like. In particular, the method 20 involvesdeflecting a test coupon as an indication of whether a mechanicalcharacteristic, such as fatigue performance, of a coated part meets aspecified level. Thus, the method 20 avoids destruction of the coatedpart or parts for evaluation purposes. The method 20 includes acalculation step 22, a deflection step 24, an evaluation step 26 and,optionally, a co-deposition step 28.

The test coupon may be produced in a co-deposition process such that asubstrate of the test coupon and substrate of the part or parts arecoated under the same conditions at approximately the same time. Thecoating of the test coupon is therefore representative of the coating onthe coated part. As may be appreciated, the co-deposition step 28 mayoccur separately from the calculation step 22, deflection step 24, andevaluation step 26, depending on whether a particular manufacturer orother party is responsible for verifying performance of the coated part.

The test coupon may include a generally rectangular steel substrate withthe coating deposited on a top surface of the substrate. The steelsubstrate may be about 25 mm wide, 102 mm long, and 1 mm thick. Thecoating may be about 0.02-0.2 mm thick, but this disclosure may also beapplied to coating thicknesses outside of the example range. In someexamples, the substrate may be made of a different material, or may beselected to match the composition of the substrate of the coated parts.Given this description, one of ordinary skill in the art will recognizeother suitable geometries for test coupons to meet their particularneeds.

The coating may be a hard coating relative to the substrate forproviding wear or erosion resistance. For instance, the coating mayinclude cobalt and phosphorous that functions as a protective sheath forthe underlying substrate. In some examples, the coating may additionallyinclude hard particulates within a matrix of the cobalt and phosphorous.

A predetermined strain threshold value ε may be used to verify fatigueperformance of the coated part using the test coupon. For a particularpart geometry and type of coating, the predetermined strain thresholdvalue ε may be known from experimental data, mechanical calculations, orthe like. The predetermined strain threshold value ε may berepresentative of other mechanical characteristics of the parts, such asfatigue performance. That is, a particular specified level of fatigueperformance has a corresponding predetermined strain threshold value ε.For strains below the predetermined strain threshold value ε, the coatedpart would be expected to meet the specified level of fatigueperformance. In other words strain threshold is directly proportional tofatigue performance as both are measuring mechanical performance of thecoating rather than a property indicating “coating quality”.

The calculation step 22 of method 20 includes calculating a deflectionvalue δ for testing the test coupon to verify whether the coated partmeets a desired fatigue performance level. The deflection value δ is afunction of the predetermined strain threshold value ε and totalthickness (t) of the test coupon, including substrate and coating. Anequation for calculating the deflection value δ may be derived based onthe geometry of the test coupon, elastic modulii of the coating andsubstrate, and known mechanical beam equations. For example, thedeflection value δ may be calculated using Equation 1 below, where k₁ isa first geometry-based constant and k₂ is a second geometry-basedconstant.

δ=ε(k ₁ −k ₂ t)   (Equation 1)

Equation 1 may be derived from the rectangular geometry of the testcoupon and by knowing the elastic modulii of the coating and substrates.Based on this data, the distance from the neutral axis of bending to thecentroid of the substrate area may be determined using Equation 2abelow, where w₁ and w₂ respectively represent the effective widths ofthe substrate and coating of the test coupon and t₁ and t₂ respectivelyrepresent the thicknesses of the substrate and coating of the testcoupon. The effective widths are determined by Equation 2b, where E₁ andE₂ are the modulus of elasticity of the substrate and coatingrespectively. The distance from the neutral axis of bending to thecentroid of the coating area may be determined using Equation 3 below.Equation 4 represents the effective Moment of Inertia, I_(eff), andEquations 5a and 5b represent known equations based on moment-areadiagrams, where I is the moment of Inertia, M is the mass and c is thedistance from the neutral axis. The combination of equations 2, 3, 4,5a, and 5b results in the composite Equation 6 below, where L₁ and L₂represent the length of the outer and inner bend points of the testfixture and where E=E₁ or the modulus of elasticity of the substrate.For a given set of geometric parameters, the deflection value δ may berepresented as a function of the predetermined strain threshold value εand thickness t of the test coupon. The deflection value δ andpredetermined strain threshold value ε may then be plotted graphicallyfor various thicknesses t of test coupons to determine the constant k₁and k₂.

$\begin{matrix}{\delta_{s} = {\frac{w_{2}t_{2}}{{w_{1}t_{1}} + {w_{2}t_{2}}}\left( \frac{t_{1} + t_{2}}{2} \right)}} & \left( {{Equation}\mspace{14mu} 2a} \right) \\{w_{2} = \frac{w_{1}E_{2}}{E_{1}}} & \left( {{Equation}\mspace{14mu} 2b} \right) \\{\delta_{c} = {\frac{t_{1} + t_{2}}{2} - \delta_{s}}} & \left( {{Equation}\mspace{14mu} 3} \right) \\{{{\frac{1}{12}w_{1}t_{1}^{3}} + {w_{1}t_{1}\delta_{s}^{2}} + {\frac{1}{12}w_{2}t_{2}^{3}} + {w_{2}t_{2}\delta_{c}^{2}}} = I_{eff}} & \left( {{Equation}\mspace{14mu} 4} \right) \\{\sigma = \frac{Mc}{I}} & \left( {{Equation}\mspace{14mu} 5a} \right) \\{\sigma = {ɛ\; E}} & \left( {{Equation}\mspace{14mu} 5b} \right) \\\begin{matrix}{\delta = {deflection}} \\{= {\left\lbrack {\frac{4\; ɛ\; {EI}}{L_{1} - {L_{2}\left( {\delta_{c} + \frac{t_{2}}{2}} \right)}} \times \frac{\left( {L_{1} - L_{2}} \right)^{3}}{48\; {EI}}} \right\rbrack +}} \\{\left\lbrack {\frac{4ɛ\; {EI}}{\left( {L_{1} - L_{2}} \right)\left( {\delta_{c} + \frac{t_{2}}{2}} \right)} \times \frac{L_{2}\left( {L_{1} - L_{2}} \right)\left( {{2L_{1}} - L_{2}} \right)}{32{EI}}} \right\rbrack}\end{matrix} & \left( {{Equation}\mspace{14mu} 6} \right)\end{matrix}$

After calculating the deflection value δ for a given test couponthickness t, the test coupon is deflected in the deflection step 24 inan amount equal to the calculated deflection value δ. A bending testfixture 42, as will be described below, may be used to deflect the testcoupon the calculated amount.

The coating of the test coupon is then evaluated in evaluation step 26as an indication of whether the fatigue performance, or other mechanicalcharacteristic, of the coated part meets a specified level. Forinstance, a dye penetrant, such as a fluorescent dye, may be applied tothe test coupon after the deflection step 24 to determine whether anycracking has occurred. In some examples, if the test coupon does crack,the cracking may produce an audible noise such that the use of a dyewould not be necessary. The occurrence of cracking may indicate that thefatigue performance or other mechanical characteristic of the coatedparts do not meet the specified level. Lack of cracking or crackingbelow a specified amount indicates that the fatigue performance or othermechanical characteristic of the coated parts meets the specified level.In this way the mechanical strength of the coating is directly measuredin comparison to a specified value as opposed to parameters which may ormay not individually affect the mechanical properties of the coating(i.e. hardness, modulus, residual stress).

FIG. 2 illustrates an example bending test fixture 42 (apparatus) fordeflecting the test coupon 40 as described above in the deflection step24 of the method 20. The test coupon 40 generally occupies a testregion. The bending test fixture 42 includes first and second supports44 a and 44 b on opposed sides of the test region. The first support 44a includes a first pair of spaced-apart support points 46, and thesecond support 44 b includes a second pair of spaced-apart supportpoints 48 that are narrowly spaced relative to the first pair ofspaced-apart support points 46. The first pair of spaced-apart supportpoints 46 project from a frame 58.

The spaced-apart support points 46 of the first support 44 a contact thetest coupon 40 near its ends, and the spaced-apart support points 48 ofthe second support 44 b contact the test coupon 40 near its center. Thespaced-apart support points 46 may be about 7.62 cm apart, and thespaced-apart support points 48 may be about 2.54 cm apart. The bendingtest fixture 42 is therefore adapted for 4-point bending. The bendingtest fixture 42 distributes the applied load over the area of the testcoupon that is between the spaced-apart support points 48. Applying theload over an area of the test coupon avoids concentrating the load on asingle point of the test coupon, which could lead to variation in theresults of the test.

A load shaft 50 is operably connected to the second support 44 b forapplying a load to the test coupon 40. It is to be understood that inother examples, the load shaft 50 may alternatively be operablyconnected to the first support 44 a.

The bending test fixture 42 includes an articulated joint 52 between theload shaft 50 and the second support 44 b for distributing a load to thesecond support 44 b. That is, there is rotational freedom between thesecond support 44 b and the load shaft 50. For example, the secondsupport 44 b includes a socket 54 and the end of the load shaft 50includes a ball 56 that is received into the socket 54. In this case,the ball 56 is a semi-sphere. Alternatively, the second support 44 mayinclude the ball 56 and the end of the load shaft 50 may include thesocket 54.

The socket 54 may have a V-shaped cross-section such that the sides ofthe ball 56 contact each side of the V-shape when the load is applied.The articulated joint 52 provides the benefit of evenly distributing theload between the spaced-apart support points 48 of the second support 44b to avoid concentrating too much of the load on one point andpotentially negatively influencing the outcome of the test.

As illustrated in the side view of the bending test fixture 42 of FIG.3, the first and second supports 44 a and 44 b, and the load shaft 50may generally be supported in the frame 58. The frame 58 may alsosupport a gauge 60 having a probe 62 that extends near the test regionto detect an amount of deflection of the test coupon 40. In thisexample, the gauge 60 is a dial indicator that indicates the amount ofdeflection. However, in other examples, other types of gauges, includingelectronic gauges, may be used.

Optionally, the bending test fixture 42 may also include a manual crank70 operably connected with the load shaft 50 for applying a load to thetest coupon 40. In operation, a user may apply a load using the manualcrank 70 such that the bending test fixture 42 deflects the test coupon40 to a desired amount that is equal to the calculated deflection valueδ. For instance, the user may apply only enough force to result in thedesired amount of deflection by watching the gauge and turning themanual crank 70 slowly enough to ramp up the deflection to the desiredamount.

A user may utilize the bending test fixture 42 to test one or more testcoupons 40 as an indication of whether a fatigue performance of one ormore coated parts meets a specified level. That is, if the user appliesan amount of deflection equal to the calculated deflection value δwithout the test coupon 40 cracking, the parts that were coated alongwith the test coupons 40 are expected to meet the specified level offatigue performance. Thus, the test coupons 40 non-destructively verifythe performance of the coated part.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this disclosure. The scope of legal protection given tothis disclosure can only be determined by studying the following claims.

1. A bending test fixture for test coupons, comprising: a test regionfor receiving a test coupon; first and second supports on opposed sidesof the test region, wherein one of the first or second supports includesa first pair of spaced-apart support points and the other of the firstor second supports includes a second pair of spaced-apart support pointsthat are narrowly spaced relative to the first pair of spaced-apartsupport points; a load shaft for applying a load to the second support;and an articulated joint between the load shaft and the second supportfor uniformly distributing the load to the first support.
 2. The bendingtest fixture as recited in claim 1, further including a gauge having aprobe extending near the test region for detecting an amount ofdeflection of the test coupon.
 3. The bending test fixture as recited inclaim 2, wherein the gauge includes a dial indicator for indicating anamount of deflection.
 4. The bending test fixture as recited in claim 1,wherein the articulated joint includes rotational freedom between theload shaft and the second support.
 5. The bending test fixture asrecited in claim 1, wherein one of the second support or the load shaftincludes a ball end and the other of the second support or the loadshaft includes a socket, the ball end and the socket together formingthe articulated joint.
 6. The bending test fixture as recited in claim5, wherein the socket includes a V-shaped cross-section.
 7. The bendingtest fixture as recited in claim 1, further comprising a manual crankoperatively connected with the load shaft.